The requirements for increasingly high areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), medium noise, i.e., signal-to-noise ratio (SNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and comprises a substrate 10, typically an (Al)-alloy, such as an Al-magnesium (AlMg) alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, ceramic and glass-ceramic materials, as well as graphite. There are typically sequentially sputter deposited on each side of substrate 10, underlayer 11, 11', such as Cr or a Cr alloy, a magnetic layer 12, 12', such as a cobalt (Co)-based alloy, and a protective overcoat 13, 13', such as a carbon-containing overcoat. Typically, although not shown for illustrative convenience, a lubricant topcoat is applied on the protective overcoat 13, 13'.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particular for growing fine grains of hexagonal close packed (HCP) Co alloys deposited thereon.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films, which are the underlayer of choice in conventional magnetic recording media. Li-Lien Lee et al., "NiAl Underlayers For CoCrTa Magnetic Thin Films", IEEE Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994. Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer is too low for high density recording, e.g. about 2,000 Oersteds (Oe).
Lee et al. subsequently reported that the coercivity of a magnetic recording medium comprising a NiAl underlayer can be significantly enhanced by depositing a plurality of underlayers containing alternative NiAl and Cr layers rather than a single NiAl underlayer. Li-Lien Lee et al., "Effects of Cr Intermediate Layers on CoCrPt Thin Film Media on NiAl Underlayers," Vol. 31, No. 6, November 1995, pp. 2728-2730.
Li-Lien Lee et al. were able to obtain an underlayer exhibiting a (200)-dominant crystallographic orientation by initially depositing a Cr sub-underlayer directly on the non-magnetic substrate at a high temperature of about 260.degree. C. using radio frequency (RF) sputtering. However, it is very difficult to obtain a Cr (200)-dominant crystallographic orientation, even at elevated temperature such as 260.degree. C., on glass, ceramic and glass ceramic substrates using direct current (DC) magnetron sputtering, which is widely employed in the magnetic recording media industry.
Li-Lien Lee et al. subsequently reported that an underlayer structure exhibiting a (200)-dominant crystallographic orientation was obtained by depositing a magnesium oxide (MgO) seedlayer using radio frequency (RF) sputtering. Li-Lien Lee et al., "Seed layer induced (002) crystallographic texture in NiAl underlayers," J. Appl. Phys. 79 (8), Apr. 15, 1996, pp. 4902-4904; and David E. Laughlin et al., "The Control and Characterization of the Crystallographic Texture of the Longitudinal Thin Film Recording Media," IEEE Transactions on Magnetics, Vol. 32, No. 5, September 1996, pp. 3632-3637. Such a magnetic recording medium, however is not commercially viable from an economic standpoint, because sputtering systems in place throughout the industry making magnetic recording media are based upon direct current (DC) sputtering. Accordingly, RF sputtering an MgO seedlayer is not economically viable. The use of an NiAl underlayer is also disclosed by C. A. Ross et al., "The Role Of An NiAl Underlayers In Longitudinal Thin Film Media" and J. Appl. Phys. 81(a), P.4369, 1996.
Conventional practices in manufacturing magnetic recording media comprise DC magnetron sputtering and high temperatures in order to obtain Cr segregation in Co-alloy grain boundaries to achieve high Hr and high SNR. Conventional practices, therefore, employ a high substrate heating temperature, e.g. above about 200.degree. C., e.g. about 230.degree. C. to about 260.degree. C., in order to achieve a desirably high Hr. However, such high substrate heating temperatures result in a reduced S* and, hence, increased medium noise.
Accordingly, there exists a need for high density magnetic recording media exhibiting high Hr and high S*. There also exists a need for efficient methodology for producing magnetic recording media exhibiting high Hr and high S*.